U.S. Pat. No. 3,962,866 discloses a two-step lambda control method wherein the difference is formed between the signals of a probe ahead of the catalyzer and another probe behind the catalyzer. As long as the catalyzer is still new, the signal supplied by the rearward probe hardly oscillates notwithstanding the oscillation which can be seen at the forward probe because of the two-step control. The difference between the two signals takes on ever increasing values. When the catalyzer exhibits a deteriorating conversion with increasing age, then the rear probe also measures an oscillation of the lambda value. For this reason, the maximum difference between the signals from the two probes becomes increasingly less with increasing catalyzer age. A warning signal is emitted when this difference drops below a threshold value. The signal from the rearward probe is not utilized for control purposes.
German published patent application DE 3,500,594 A1 and U.S. Pat. No. 4,622,809 disclose two-step lambda control methods which have measurement signals from a measurement probe arrangement ahead of the catalyzer for controlling the lambda value and residual signals from a test lambda probe behind the catalyzer for evaluating the conversion rate of the catalyzer. The control parameters with which the two-step control is driven are continuously so adjusted that the amplitude of the test signal is minimal and the mean value of the test signal moves in a pregiven range. A fault signal is emitted when the minimal attainable amplitude of the test signal exceeds a threshold value or when the mean value of the test signal moves out of the pregiven window. The fault signal indicates that the catalyzer no longer adequately performs its function.
U.S. Pat. No. 4,884,066 discloses a system wherein a warning signal is then emitted when the amplitude of the signal of a probe behind the catalyzer becomes too high. In this method, there is therefore no investigation as to whether a large signal amplitude of the probe behind the catalyzer is caused by a large lambda value amplitude ahead of the catalyzer. This method is therefore the simplest of those described but is also the method most prone to error. The signal of the probe behind the catalyzer is not utilized for open-loop or closed-loop control purposes; instead, the signal is only used to generate the above-mentioned warning signal.
German published patent application DE 3,500,594 A1 and U.S. Pat. No. 4,622,809 describe methods which can relatively easily lead to control oscillations which are difficult to control because of the feedback in the lambda control loop and the feedback in the test signal loop which acts on the lambda control loop via the control parameters. This is especially then the case when, because of a disturbance, the amplitude of the test signal is temporarily relatively large. Then, the values of the control parameters are so changed that only low amplitudes are present ahead of the catalyzer and therefore also behind the catalyzer. The control becomes more sluggish in this way whereby it is more difficult to eliminate the disturbance which has caused this effect. In this way, an especially large deviation from the desired value in the lambda control loop can settle in because of the action of the test signals which indeed should be prevented. The advantage of being able to limit the amplitudes of control oscillations with increasing catalyzer age is connected with the disadvantage that unnecessarily high control deviations can develop because of the overall sequence of the method as long as it is not necessary to change the control parameters in view of the excellent operation of the catalyzer.